Incorrect premiums debited

Virgin Active has been debiting me incorrect full membership premiums for my son that turned 18 and was always on Discovery Vitality as a dependent. Made numerous phone calls, sent emails and also visited the Randburg Gym, dealt with over 20 different consultants and they are all useless!!! Not sure what pathetic system they use even after they noticed the fault and error from their side. Consultants have no knowledge about the product, premiums, contract procedures and terms and conditions. The excuse they have is to come complete new paperwork every time when they are already linked with Discovery Vitality. Why is this so hard to rectify such a simple query?!!! Urgent feedback from somebody that's clued up please!!!